The present invention relates generally to the fields of organic chemistry and optically anisotropic coatings. More specifically, the present invention is related to methods of synthesizing heterocyclic sulphoderivative compounds and manufacturing optically anisotropic coatings based on these compounds.
Modem technological progress requires development of optical elements based on new materials with specific, controllable properties. In particular, the necessary element in modem visual display systems is an optically anisotropic film that is optimized for the optical characteristics of an individual display module.
Various polymer materials are known in the prior art for use in the production of optically anisotropic films. Films based on these polymers acquire optical anisotropy through uniaxial extension and coloring with organic dyes or iodine. Polyvinyl alcohol is one commonly used polymer in this application. However, the low thermal stability of films based on polyvinyl alcohol limits their applications. Polyvinyl alcohol based films are described in greater detail in Liquid Crystalsxe2x80x94Applications and Uses, B. Bahadur, ed., Vol. 1, World Scientific, Singapore, N.Y. Jul. 1990, p101. It is desirable to develop improved methods of forming anisotropic films with the characteristics of greater heat resistance and more convenient synthesis and film formation.
Organic dichroic dyes are a new class of materials currently gaining prominence in the manufacture of optically anisotropic films with desirable optical and working characteristics. Films based on these materials are formed by coating a liquid crystal (LC) aqueous solution of supramolecules formed by dye molecules on a substrate surface following water evaporation. The produced films are imbued with anisotropic properties either by preliminary mechanical ordering of the underlying substrate surface as described in U.S. Pat. No. 2,553,961 or by applying external mechanical, electromagnetic, or other orienting forces to the coating on a liquid crystal substrate material as described in pending PCT Application No. WO 94/28073.
Liquid crystal properties of dye solutions are known in the prior art. However, investigations into their application in this manner is a more recent development in the past several years. Recent studies into-these phenomena have been motivated largely by industrial applications in liquid crystal displays (LCDs) and glazing. Supramolecules form a lyotropic liquid crystal (LLC). Substantial ordering of dye molecules in columns allows use of these mesophases to create oriented, strongly dichroic films. Dye molecules that form supramolecular liquid crystal mesophases are special. They contain functional groups located at a molecule periphery that determine the water solubility of the dye. Organic dye mesophases are characterized by specific structures, phase diagrams, optical properties and dissolving capabilities as described in greater detail in Jean-Marie Lehn, Supramolecular Chemistry, N.-Y., 1998.
Anisotropic films characterized by high optical anisotropy may be formed from LLC systems based on dichroic dyes. Such films exhibit both the properties of E-type polarizers, due to light absorption by supramolecular complexes, and the properties of retarders. Retarders are films with phase-retarding properties in those spectral regions where absorption is lacking. Phase-retarding properties of the films are determined by their double refraction properties: different refraction indices in the direction of LC solution deposition and the direction orthogonal to the deposition direction. If high-strength dyes are used for the film formation, the films are also characterized by high thermal and photo stability.
Extensive investigations aimed at developing new methods of creating dye-based films through manipulation of deposition conditions are currently underway. Of additional interest is the development of new compositions of lyotropic liquid crystals (LLC). New LLC compositions may be developed through the introduction of modifying, stabilizing, surfactant and other additives to known dyes, thus improving film characteristics. More detailed discussions of these processes are provided in Russian patent No. RU 2047643 and published PCT patent application No. WO 99/31535.
The requirements for producing anisotropic film with improved selectivity in different wavelength ranges are increasing. Films with different absorbance maxima located in a wide spectral range from the infrared to the ultraviolet are needed in a wide range of technology areas. The requirements have led to development of an expanding assortment of compounds capable of forming LLC phases and films with required properties.
Much recent research attention has been directed to the materials used in the manufacturing of double refraction films (retarders), which are used in LC displays and telecommunications applications, such as, for example, those described by P. Yeh, Optical Waves in Layered Media, New York: John Wiley andSons, Inc, 1998 and P. Yeh, and C. Gu, Optics in Liquid Crystal Displays, New York: John Wiley andSons, Inc., 1999. It has been found that ultra-thin double refraction films may be produced using known technologies to form optically anisotropic films composed of organic dye LLC systems. Recent reports on manufacturing of thin crystalline optically anisotropic films based on disulfoacids of the red dye Vat Red 14 (Lazarev, P. and Paukshto, M., xe2x80x9cThin Crystal Film Retarders,xe2x80x9d 2000, Proceeding of the 7th International Display Workshops, Materials and Components, Kobe, Japan, p. 1159-1160), have identified the mixture of naphthalenetetracarboxylic acid dibenzimidazole cis- and trans-isomers of the following structures: 
The prior art technology permits control of the direction of the crystallographic axis of a film during coating and crystallization on a substrate. Films were formed on glass plates measuring approximately 5 cm by 7.5 cm. The deposited films were uniform in composition and had high a crystal ordering with a dichroic ratio, Kd, of approximately 28. These films may be used either as polarizers or as retarders.
Oriented red dye based films typically exhibit high anisotropy characterized by a large difference in the refraction indices: xcex94n=noxe2x88x92ne and falls in the range of approximately 0.6 to 0.8 at wavelengths in the range of approximately xcex=550 to 700 nm. However, their application is limited, because the films work as retarders only in the green spectral region where the dye does not absorb.
Thin, double refraction films based on sodium chromoglycate (DSCG) with the following structure may also be prepared to be transparent in the visible region: 
The anisotropy of the oriented film formed from this compound is not very highxe2x80x94the difference in the refraction indices is in the range of approximately 0.1 to 0.13. However, the thickness of films based on DSCG may be varied in a wide range, thus allowing preparation of films with a desired phase-retarding effect despite somewhat low anisotropy. More details on these films are provided by Lazarev, P. and Paukshto, M., xe2x80x9cThin Crystal Film Retardersxe2x80x9d (2000), Proceeding of the 7th International Display Workshops, Materials and Components, Kobe, Japan, p. 1159-1160. The main disadvantage of these films lies in their dynamic instability which leads to gradual recrystallization and anisotropy degradation.
Various blends based on water-soluble organic dyes used for forming anisotropic films according to described above technology are known in the prior art. Examples are described in published PCT patent applications No. WO 94/28073 and No. WO 99/31535. However, the main drawback of these materials is their high absorbance in the visible spectral region which limits their application for forming transparent double refraction films.
It is an object of the present invention to provide a series of new chemical compounds, namely phenanthro-9xe2x80x2,10xe2x80x2:2,3-quinoxaline sulfoderivatives capable of forming stable lyotropic liquid crystals and producing anisotropic, at least partially crystalline films based on these compounds. The present invention is further directed to expansion of the assortment of compounds that are either not absorbing or only weakly absorbing in the visible spectral region and that are capable of forming a LLC phase with enhanced stability for producing anisotropic, at least, partially crystalline films with high optical characteristics at simultaneous elimination of mentioned above drawbacks.
One embodiment of the present invention provides a method of producing slightly-colored films that may be used both for polarizing and for double-refraction. Phenanthro-9xe2x80x2,10xe2x80x2:2,3-quinoxaline and its derivatives do not have as developed a xcfx80-electron conjugation system as do other known dichroic dyes, and they therefore absorb in the UV and near visible spectral regions. Thus, the molecule of unsubstituted phenanthro-9xe2x80x2,10xe2x80x2:2,3-quinoxaline has an absorption maximum at xcex≈390 nm.
Another embodiment of the present invention provides LLC phases with enhanced stability over broad concentration, temperature and pH ranges. These LLC phases are formed from organic compounds that ease the film formation process, and use standard equipment for coating thereby facilitating production of films with reproducible parameters.
Yet another aspect of the present invention provides organic compound mixtures in which the optimal hydrophilic-hydrophobic balance is realized. This in turn affects the size and shape of the resultant supramolecular complexes and the molecular ordering rate in the complex itself, thus allowing attainment of the required solubility of the considered compounds. Proper solubility values promote high stability of the produced LLC phases. Reproducibility of the formed film parameters is enhanced in this manner, and thereby the film formation process is eased by reducing the requirements for choosing and controlling technological conditions at different stages of the film formation. Furthermore, the optical performance of the produced films is improved by increased uniformity of alignment on a substrate of the planes of phenanthro-9xe2x80x2,10xe2x80x2:2,3-quinoxaline sulfoderivative molecules. In addition, the dipole moments of electronic transitions with respect to direction lie in these planes and are determined by external orienting stresses.